1. Field of the Invention
The present invention relates to an implantable medical device of the type used for measuring ventricular pressure in a subject.
2. Description of the Prior Art
The pressure of the blood entering the heart is of great interest. All the blood from the veins in the body enters the heart into the right atrium. This represents 95% of the total venous blood volume, the remaining 5% of the volume enter from coronary sinus, which is the retum from the hearts own blood supply. The pressure in the vena cava, the large vein just outside the heart, is called central venous pressure (CVP). The average level of CVP is just a few mmHg but because the vena cava is very elastic (has high compliance), a small change in pressure indicates that a large volume of blood is involved. The CVP is therefore of great interest because it is an indicator of the blood volume that flows through the veins and enters the heart. The pressure in the vena cava will increase if the heart beats too weakly. The increase indicates that the blood is backed up in the veins. The normal response from the heart in this situation is to beat faster and/or increase the stroke volume. There is also another factor that can cause an increase in the CVP resulting from the increase in blood volume when a person lies down, e.g. when he goes to bed at night. The response of the heart is the same as above, i.e. to beat faster and/or increase the stroke volume.
In U.S. Pat. No. 5,040,540 different methods of measuring central venous pressure are disclosed. To obtain a valid central venous pressure a measurement catheter could be placed within the right atrium or one of the great veins of the thorax (e.g. the superior vena cava, the innominate vein or the subclavian vein).
Measuring in the right atrium should be avoided according to U.S. Pat. No. 5,040,540 due, inter alia, to the risk of perforation of the atrial wall.
Pressure sensors adapted to be inserted inside a heart are well known in the art, see e.g. U.S. Pat. Nos. 5,843,135 and 5,324,326.
In U.S. Pat. 5,843,135 a piezoelectric pressure transducer is arranged in a patient's heart, e.g. in the right ventricle or right atrium.
U.S. Pat. No. 5,324,326 discloses a pressure sensing pacing lead having a distal pressure sensor for sensing hemodynamic pressure within the heart. The pressure sensor has an integrated circuit chip having a layer of piezo-resistive material and a non-conductive base member.
During diastole, the filling phase of the heart cycle, the tricuspid valve, which is the valve between the right atrium and the right ventricle of the heart, is open. A consequence thereof is that pressure measured in the right ventricle during diastole also reflects the pressure in the right atrium and also the pressure close to the heart in the veins transporting blood into the right atrium (superior vena cava etc.).
U.S. Pat. No. 5,163,429 discloses a hemodynamically responsive system for treating a malfunctioning heart. A signal is developed that is representative of pressure sensed at a site in a patient's circulatory system. This signal may represent e.g. short-term mean right ventricular pressure, mean central venous pressure, right ventricular systolic pressure, or right ventricular diastolic pressure.
In U.S. Pat. No. 5,163,429 as well, a signal representative of the right ventricular systolic pressure is determined by detecting a real time representation of peak pressure provided that a zero slope condition follows a positive slope. The thus detected peak pressure is shifted into a shift register for further evaluation. Following the determination of the right ventricular systolic pressure it is briefly described that similar circuitry also may be used to determine right ventricular diastolic pressure by using a negative slope detector instead of a positive slope detector. According to the system in U.S. Pat. No. 5,163,429 only a single pressure value (the minimum value) is determined each heart cycle during the diastolic phase. The determined pressure value is then used to obtain short-term or long-term signal representations of right ventricular diastolic pressure.
U.S. Pat. No. 5,368,040 discloses an apparatus and method for monitoring and measuring a number of hemodynamic variables from a single, chronically implanted absolute pressure sensor.
In this known device the first and second derivatives of the pressure signal are used together with the ECG signal to identify start and end points of the systolic and diastolic intervals, respectively.
As shown in FIG. 1 in U.S. Pat. No. 5,368,040 the PA systolic pressure is determined by feeding the sensed RV pressure sensor output into a sample and hold circuit that is enabled by the sensing of the R-wave. The systolic pressure is then latched when dP/dt goes negative. The latched value is then held until the next R-wave is sensed.
One drawback with the apparatus described in U.S. Pat. No. 5,368,040 is that information related to the internal EGM signal is required in order to identify specific portions of the heart cycle which renders the apparatus complicated.
A general drawback with the above-described prior art systems is that only limited information of the pressure variation is obtained. No continuous pressure curve of the diastolic pressure is determined.